What would happen if the 25th gets revoked and say Trump does get impeached or falls ill and can not be president. Would we then vote for president for the years the active president would have had left in his term?
Without the 25th amendment, there would be several ambiguities. If the President was impeached, it would be relatively clear. The powers of the presidency would devolve to the Vice President. There is (and was) disagreement among scholars about whether the Vice President actually became President or merely exercised the powers of the presidency but that is a rather academic matter-- for all intents and purposes, the Vice President would be the President for the remainder of the term. The 25th amendment clarified that the Vice President actually became President. Prior to the 25th amendment, there was no provision to replace the Vice President in that case. Presidential succession would then be controlled by the Presidential Succession Act which would put the Speaker of the House next in line even if that person was from a different party. Today, that would mean that Pence would assume the duties of the President and Pelosi would be next in line if Pence were unable to finish the term.
If the President became ill but did not die, things become stickier. If the President agrees that he was too ill to carry out his duties, power would devolve to the Vice President either permanently (should the President resign) or temporarily (as happens if, say, the President needs surgery and will be incapacitated for a period of time). In the past, though, even when presidents were deathly ill, they generally did not devolve power. Edith Wilson realistically exercised the power of the presidency after Woodrow's stroke, for example, rather than devolving power to the Vice President. Without the 25th amendment, there was no way to compel an ill president to hand over power unwillingly. That could obviously have created a constitutional crisis which is one of the reasons the amendment was adopted.
In no case would an election be held early. If the Vice President was unable to complete the President's term, you'd just go down the line of succession in the Presidential Succession Act.
A good overview of the 25th amendment and the line of succession before the 25th amendment is here.
This is impossible to answer, because it depends on how it gets revoked (or, more accurately, repealed). The only way to remove a Constitutional amendment is with another amendment, and that amendment will specify what (if anything) will replace the 25th.
As an example, the 21st Amendment, which repealed the 18th Amendment and replaced it with something that had very similar wording, but with a significantly more permissive law.
18th Amendment (section 1, emphasis mine):
... the manufacture, sale, or transportation of intoxicating liquors within, the importation thereof into, or the exportation thereof from the United States and all the territory subject to the jurisdiction thereof for beverage purposes is hereby prohibited.
21st Amendment (section 2, emphasis mine):
The transportation or importation into any State, Territory, or possession of the United States for delivery or use therein of intoxicating liquors, in violation of the laws thereof, is hereby prohibited.
So it's useless to speculate as to what would happen if the 25th Amendment gets repealed, because all we can say is that "Without the 25th Amendment, the process that replaced it would be followed."
The 25th Amendment has 4 sections. I'm not sure which section(s) (or all of them?) that you are considering might be repealed.
Section 1 codifies the Tyler Precedent that the Vice President actually becomes President instead of just acting as President when the President dies or resigns or is impeached. Article 2, Section 1, Clause 6 of the US Constitution isn't clear whether the Vice President becomes President, or just acts as President, in those cases. If Section 1 is repealed, it would just go back to a situation where it's ambiguous, but it would still be widely accepted that the Vice President becomes President in those cases.
Section 2 provides a procedure for filling Vice Presidential vacancies. If it is repealed, it would go back to the situation where Vice Presidential vacancies cannot be filled, and so the Vice Presidency can go on being vacant for sometimes years (as has happened several times in US history before the 25th Amendment), and it would increase the probability that the Speaker of the House would have to to act as President at some point.
Sections 3 and 4 provide mechanisms for the President, or other people around him, respectively, to declare that the President is unable to discharge the powers and duties of his office, which causes the Vice President to be Acting President; and they also provide mechanisms to declare that the President is able to discharge the powers and duties of his office again. Note that even without these sections, Article 2, Section 1, Clause 6 of the US Constitution already provides for the Vice President to act as President when the President is unable to discharge the powers and duties of his office, but it doesn't specify how to determine whether the President is unable to discharge them or not. Sections 3 and 4 of the 25th Amendment provide procedures for this inability to officially begin and end. If these sections are repealed, there could be constitutional crisis if one party in the government claims that the President is unable to discharge his powers, while another party claims that he is, with no agreed-upon way to resolve it.